Some radio transmitting systems employ the possibility of adding in space the signals concurrently sent out by two separate transmission sources. In order that the resulting field be able to represent the sum of the fields of the two transmission sources, it is essential that the phases of the high-frequency waves constituting the carrier wave and the sidebands, for example in the case of radio-navigation signals, correspond to each other with fidelity. The radio-navigation transmitting systems known as V.O.R. or I.L.S. employ phase-controlling devices. Such devices such as that shown in FIG. 1 comprise a pilot oscillator 1 emitting simultaneously over two amplifying channels a signal whose frequency is equal to the frequency of the carrier wave. An amplifier 2, possibly including an amplitude-modulating stage, delivers the carrier wave in the first channel whereas a modulating amplifier 3 preceded by a phase-shifting device 4 delivers a second signal whose frequency is identical to that of the carrier wave and whose phase is fixed with respect to the latter. A phase comparator or mixing stage 5 connected to the outputs of the amplifiers 2 and 3 delivers, after filtering of the difference signal, a continuous voltage or error signal which exactly measures the phase difference between the high-frequency signals delivered by the two amplifiers. The error signal, after amplification by a direct-current amplifier 6, is used to control the analog phase shifter 4 and to lock the signals delivered by the amplifier 3 in phase with those delivered by the amplifier 2. The high-frequency output signal of amplifier 3, phase-controlled by the carrier wave S.sub.1 issuing from amplifier 2, is fed to a sideband generator 7 controlled in a conventional manner by a low-frequency modulation signal BF to produce a sideband signal S.sub.2.
Such devices permit a stabilization of the phase of the high-frequency signal S.sub.2 only if the sideband generator 7 has a very high intrinsic stability since that generator is not included in the regulating loop. This intrinsic stability of the sideband generator requires an operation of the latter at a high level. The incorporation of the sideband generator in the regulating loop cannot be achieved directly with such devices, the phase of the high-frequency signals S.sub.2 being inverted by .pi. every semi-period of the low-frequency modulation signal BF. The direction of the phase drift can therefore not be determined.